In a gas turbine engine fuel delivery system, pump assemblies, as shown for example in US 2005/0232784, are typically used for pumping the fuel. Where such assemblies include gear pump, gear elements are commonly supported by bearing blocks which are adapted to receive respective bearing shafts of the gears through a bore of each bearing block. These bearing blocks also typically abut axially-directed faces of respective gears of the pumps. The bearing blocks may be for solid bearings, or pressure loaded bearings. A solid bearing typically transfers load from journals to the pump housing, and additionally can transfer axial load to the housing. Pressure loaded bearings also transfer load from journals to housing, and in addition can provide an axial force and a moment against the axially-directed face of the gear which the bearing block abuts.
It is known to use bimetallic (alloy) bearing blocks, as shown, for example, in U.S. Pat. No. 4,523,365. Such a bimetallic bearing block generally comprises an inner bush covered with an outer backing layer. The inner bush is formed of an alloy which provides a tribologically compatible surface for the gear side face and journal to run against. However it may be undesirable for the whole bearing block to be formed of such an alloy. Two reasons for this are that firstly the weight of a block formed solely of such an alloy may be larger than desired, and secondly the difference between the coefficient of thermal expansion (CTE) and that of the light alloy normally used for the pump housing body may be large. Therefore, in a bimetallic bearing block the inner bush is coated with a light alloy backing layer, which reduces the overall weight of the block, and mitigates the CTE difference with the pump housing body.
In order to provide a tribologically compatible surface for the gear side face and journal to run against, an antifriction alloy is typically used to form the inner bush. The antifriction alloy may be, for example, a lead bronze alloy. In particular, the antifriction alloy may be a high lead bronze alloy, with a lead concentration of typically 20-30 wt %, although the alloy composition is not limited to this range, and may be selected according to the desired material properties required for the bearing block and the particular application for which it is being used.
However, testing of bearing blocks which use such an antifriction alloy has shown that such blocks can be prone to suffer permanent radial deformation along the bearing bore when operated above a threshold pressure/temperature combination. If this permanent radial deformation is significant, it can reduce the clearance between the gear journal and the bearing bore. As a result of this reduced clearance, overheating can occur, which may result in mechanical damage of the gear and/or bearing. This problem is more evident in bearing blocks having a higher concentration of lead (e.g. 30 wt % lead bronze bearings), although the same problem may also occur to some extent in bearings having a different alloy composition (e.g. in 20 wt % lead bronze bearings) or made from a different antifriction alloy.
One possible solution to the problem of preventing radial deformation is to use materials having increased strength. Lead-bronze alloys which have a lower lead content, e.g. 20 wt % lead bronze, have a higher yield stress and tend to suffer less permanent deformation in use than lead bronze alloys with a higher lead content, e.g. 30 wt % lead bronze. However, bearings made of 20 wt % lead bronze have also been shown to have poorer performance as thrust bearings than those manufactured from 30 wt % lead-bronze, and suffer a problem of poor load carrying capacity at the thrust face of the bearing block. One reason for this poorer performance may be due to the relationship between thermal conductivity of the material and the wear properties of the material.
FIG. 1 shows a typical Stribeck curve, which describes how coefficient of friction varies for different lubrication regimes. It can be seen that typically, higher coefficients of friction occur in a mixed mode lubrication regime as compared to in a full-film lubrication regime. The thrust faces in a gear pump bearing arrangement typically operate at least partly in a mixed-film lubrication regime. This is evident from the wear and scoring that can be visualised at the thrust surfaces after running. In contrast, the journal element of the bearing typically operates in a full-film lubrication regime. Generally, the loads on the thrust face of a bearing block are lower than those acting on the journal element of the bearing, but the sliding velocities are higher. Accordingly, local heating of the thrust face and gear side face is more likely than in the journal element of the bearing. The situation may be particularly acute at the region of the thrust face under the gear root circle diameter where fluid cooling is limited by the restricted fluid flow that occurs across this section of the thrust face. The primary mode for heat transfer away from the thrust face at this point may be conduction through the inner bush and backing layer of the bearing block. Therefore, depending on the thermal conductivity of the alloys of these components, there may be a problem that heat cannot transfer away from the thrust face sufficiently quickly. This may then lead to reduced wear characteristics, and correspondingly poorer performance of the thrust face in a thrust bearing capacity.